Why does my Apple phone always send a short message followed by an identical short message?

This situation should be a mobile phone problem. You can prove that you sent a message after you put the card on another mobile phone to see if there is a similar situation (if there is a card problem). If not, it should be a problem with mobile phone software. (or mobile phone settings) I suggest you go to the mobile phone repair center (because I haven't seen your mobile phone, I can't give you a very accurate question). Glad to answer your question, I hope it can be adopted!